A conventional electrical amplifier is essentially a variable resistance that uses energy from a direct current (DC) source to increase alternating current (AC) energy. A parametric amplifier uses a nonlinear variable reactance, such as capacitance that varies with time, to supply energy from an AC source to a load. The energy required to vary the capacitance is obtained from an electrical source called a pump. Since reactance does not add thermal noise to a circuit, parametric amplifiers produce less noise than most conventional amplifiers.
Dynamic properties of mechanical resonators can be utilized to create a parametric amplifier. Parametric amplification has long been used as a technique for making a low noise electronic amplifier. The amplification of the applied signal is done by making use of energy from the pump. Parametric amplifiers with a variable-capacitance main-oscillator semiconductor diode are used in radar tracking and communications between Earth stations, satellites, and deep-space stations. The noise temperature of cooled amplifiers is in the range of 20 to 30 K, and the gains are up to 40 dB. This type of amplification is also widely used in optics as well as in electronic traveling wave applications. Parametric amplification is also observed in microelectromechanical systems (MEMS).
Scaling down the dimensions of MEMS into the micron and submicron region shortens the response time down to nanoseconds. A bar clamped at both ends with dimensions 0.2×0.2×2 microns microfabricated from single-crystal silicon exhibits a resonant frequency of 380 MHz, extending the area of MEMS applications into the ultra-high frequency (UHF) region. UHF MEMS devices are expected to replace bulky and power-hungry elements in telecommunication devices, such as quartz oscillators, filters, frequency converters, etc. Since the process of MEMS fabrication is compatible with modern silicon technology, micromechanical devices can be the basis for next generation UHF integrated circuits.
However, the way to transform an electrical signal into mechanical motion and vice versa represents one of the main challenges in MEMS applications. In the most straightforward filter configuration, a MEMS oscillator would be driven electrostatically by an external voltage Vext (signal from antenna), exhibiting mechanical vibrations when Vext has a resonating frequency component. Such an UHV mechanical vibration with nanometer amplitude must be converted back into an electric signal for further processing.
The high frequency of the mechanical motion practically excludes supersensitive but slow detection methods, such as electron tunneling, used in accelerometry or magnetometry. Capacitive and optical detection methods are considered as the most suitable because of their fast response and high sensitivity. An optical method employing a focused laser beam allows a design not overloaded by closely placed electrodes, and also provides reduced cross-talk between driving and detection signals. Interferometric and beam-deflection techniques convert the intensity variation of the reflected laser beam (caused by mechanical motion) into electrical signal with subsequent amplification and measurement by an electric circuit.
Laser wavelength puts a limit on the sensitivity of the interferometric method, making detection of nanometer motion problematic. Signal processing (amplification in this case) on the mechanical level, provided before the mechanic-to-electric conversion by an active UHF micromechanical component, can solve such a problem and is considered as a key point for future MEMS devices.
Parametric amplification represents a means for “mechanical signal processing” in regards to MEMS oscillators. The energy, necessary to gain mechanical motion is provided by periodic modulation of the oscillator's parameter—effective stiffness k. Small mechanical vibrations, induced by a weak external force can be amplified by the parametric mechanism and the enhanced vibrations will be detected optically. Since a “mechanical parametric preamplifier” can be noise-free down to the quantum-mechanical level, it should greatly improve the signal-to-noise ratio of the resulting signal. A mechanical oscillator embedded in a degenerate parametric amplification scheme is also fundamentally interesting because mechanical squeezed states can be produced by such a system: the thermal vibration in one phase of the response can be reduced below the thermal equilibrium level. In MEMS oscillators, the only method that has demonstrated parametric amplification is achieved by modulation of the effective spring constant by superimposing a time-varying electric field between the oscillator and an additional, closely located capacitor plate.